Hydrogels are water-swellable or water-swollen materials typically having a structure defined by a crosslinked network of hydrophilic homopolymers or copolymers. The hydrophilic homopolymers or copolymers may be water-soluble in free form, but in a hydrogel, they are rendered insoluble (but swellable) in water due to covalent, ionic, or physical crosslinking. In the case of physical crosslinking, the linkages may take the form of entanglements, crystallites, or hydrogen-bonded structures. The crosslinks in a hydrogel provide structure and physical integrity to the network.
Hydrogels may be classified as amorphous, semicrystalline, hydrogen-bonded structures, supermolecular structures, or hydrocolloidal aggregates. Numerous parameters affect the physical properties of a hydrogel, including molecular weight of gel polymer, type of crosslinking, and crosslinking density. The crosslinking density, for example, influences the hydrogels macroscopic properties, such as storage modulus (E′), which is a measure of the energy stored during a cycle of elongation or compression. Pore size and shape, pore density, and other factors can also impact the surface properties, optical properties, and mechanical properties of a hydrogel.
Hydrogels have been derived from a variety of hydrophilic polymers and copolymers. Polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyvinyl pyrrolidone (PVP), polyacrylamide (PA), polyhydroxyethyl methacrylate (PHEMA), and copolymers of the foregoing, are examples of polymers from which hydrogels have been made. Hydrogels have also been formed from biopolymers such as chitosan, agarose, hyaluronic acid and gelatin, as well as interpenetrating network (IPN) hydrogels such as gelatin crosslinked with poly(ethylene glycol) diacrylate.
Hydrogels have shown promise in biomedical and pharmaceutical applications, mainly due to their high water content and rubbery or pliable nature, which can mimic natural tissue and can facilitate the release of bioactive substances at a desired physiological site. For example, hydrogels have been used and/or proposed in a variety of tissue treatment applications, including as implants, tissue adhesives, bone grafts for spinal and orthopedic treatments such as meniscus and articular cartilage replacement, and intervertebral disc nucleoplasty. One drawback to the use of conventional hydrogels in certain tissue treatment applications, and in particular bone tissue treatments, is that such hydrogels are typically weak materials that fracture easily and do not have desired levels of durability and wear resistance. Devices made from PVA hydrogels have been observed to fail due to wear, such as by tearing, abrasion, or shredding.
In the context of nucleoplasty, where the nucleus pulposum of the intervertebral disc is replaced with a prosthetic, simple hydrophilic polymeric hydrogels are inadequate. These hydrogels do not possess the required compression strength or toughness needed in the intervertebral disc environment. Polyurethanes do possess the requisite toughness but are difficult to work with because they resist deformation.
Therefore, it would be beneficial to provide hydrogels and methods of making such hydrogels that are similar to polyurethanes in that they are significantly stronger, more durable, and possess improved wear characteristics compared to current hydrogels such as PA, PVA, PVP, and PVA/PVP blends, but that also possess certain characteristics of a hydrogel, namely pliability and high water absorption.
Polyurethane elastomers derive their properties from phase separation into hard (urethane) and soft (polyether or polyester) domains. For instance, spandex, which DuPont sells under the trade name LYCRA®, has both urea and urethane linkages in its backbone. What gives spandex its special properties is the fact that it has hard and soft blocks in its repeat structure. The short polymeric chain of a polyglycol, usually about forty or so repeat units long, is soft and rubbery. The rest of the repeat unit, including the urethane linkages, the urea linkages, and the aromatic groups, is extremely rigid. Thus, polyurethane consists of alternative soft and hard segments, which can self-assemble into two phases. “Phase” is traditionally defined as a homogeneous part of a heterogeneous system. Respectively, “phase separation” is the transformation of a homogeneous system to heterogeneous one.
One way of increasing the mechanical properties of a hydrogel may be to formulate a solid state, phase separated hydrogel. By analogy to polyurethane, a phase separated, branched, copolymer hydrogel may yield improved mechanical properties. However, the absorbent nature of hydrogels that is desirable for biomedical applications needs to be maintained.
Thus, there is a need for a hydrogel having increased mechanical properties that also maintains water absorbent properties and this need can be met with a solid state, phase separated, branched, copolymer hydrogel.